Shadow Burns. Preternatural Affairs, #4

When more than a dozen people die at a retirement home, the official story is carbon monoxide poisoning. Cèsar Hawke is convinced the reason is less mundane and more infernal. But that's his job. As an agent working for the Office of Preternatural Affairs, he's always looking for supernatural answers to deadly questions. Isobel Stonecrow agrees to help him find the truth. With her powers of necrocognition, she can speak to the dead and get the real story.
But when they return to the crime scene, they find a lot more than cadavers. They find a nightmare that they can't escape-a nightmare from Isobel's past, which even she can't completely remember thanks to the contract that signed away her soul. Cèsar will have to disinter Isobel's secrets to save her. He'll learn who Isobel used to be, what she's done, and the price she paid.no matter how deadly the knowledge might be.